1. Field of the Invention
The present invention relates to a fisheye lens system.
2. Description of the Prior Art
A fisheye lens element is classified into four different categories according to the projection method thereof:
(a) Orthographic Projection y=−f·sin θ
(b) Equisolid-angle Projection y=−2f·sin(θ/2)
(c) Equidistance Projection y=−f·θ
(d) Stereographic Projection y=−2f·tan(θ/2)
wherein
y designates the image height;
f designates the focal length; and
θ designates the half angle-of-view.
Accordingly, in the above projection method (a), at an image height y with the half angle-of-view of 90°, the image height is y=f.
In the above projection method (b), at an image height y with the half angle-of-view of 90°, y=−1.414f.
In the above projection method (c), at an image height y with a half angle-of-view of 90°, y=−1.571f.
In the above projection method (d), at an image height y with a half angle-of-view of 90°, y=−2f.
Furthermore, when the image height Yx at the half angle-of-view of 90° is determined, the focal length is as follows:
in the above projection method (a), f=−Yx;
in the above projection method (b), f=−Yx/1.414;
in the above projection method (c), f=−Yx/1.571;
in the above projection method (d), f=−Yx/2.
Fisheye lens systems for silver halide film cameras generally have employed the above projection methods (a) and (b). However, in recent digital cameras, due to improvements in image processing technology, a fisheye lens system employing a projection method which has a stronger resolving power at the peripheral portion, e.g., the above projection method (d), has been proposed, as shown in Japanese Unexamined Patent Publication No. 2000-221391.
However, in order to achieve balance between the axial resolving power and the off-axis resolving power, it is desirable to employ a projection method which is similar to the above projection method (c).